Semi-continuous electrolytic process



3,030,285 SEMI-@QN'HNUUUS ELEQTRULYTHC PRUCESS Ralph M. Sarla, Lewiston,N.Y., assignor to Union Carbide Corporation, a corporation of New YorkNo Drawing. Filed May 31, 1955, Ser. No. 512,2a7 9 Claims. (Cl. 294-64)This invention relates to an electrolytic process for the production ofextremely pure reactive refractory metals of groups IV, V and VI of theperiodic table.

On'e commonly employed method of producing such metals is by theelectrolysis of one of their compounds dissolved in an electrolyteconsisting of fused alkali and alkaline earth metal halides.Electrolytes generally em 'ployed in this connection are calciumchloride, mixtures of calcium and magnesium chlorides, mixtures ofcalcium chloride, sodium chloride and potassium chloride, as well assodium and potassium chlorides, either alone or in combination.

The inherent ductility of many of these metals is adversely affected bytraces of elemental impurities. In particular, minute amounts ofhydrogen are capable of penetrating and transforming the metallic grainstructure. Metals having this element often pass all the initialhardness and tensile strength tests, but may fail abrupt-1y after ashort service period under strain.

Reactive refractory metals absorb hydrogen from several sources. One ofthese is moisture accumulated in the brick of metal furnace duringtemporary shut-down. Another is the electrolyte used in theirproduction. Still another is live steam employed to clean dies withwhich these metals are forged.

In the specific case of the present commercial procedure for extractingtantalum from .its ores, usually tantalite, difficult and expensivefractional crystallization steps are necessary to remove both metallicand non-metallic impurities, which steps are followed by electrolysis ofpotas sium-tantalum-heptafluoride on a batch basis. During electrolysisa finely powdered, slightly impure tantalum metal is deposited at thecathode, and a portion of the fluorine present in the original salt isevolved at the anode as a highly corrosive gas. At the end of theoperation the contents of the cell are allowed to solidify, removed fromthe cellthe metal being separated by wet chemical process. Thesolidified mass remaining in the cell at the end of the operationcontains approximately percent of tantalum metal, the balance consistingof potassium fluoride and potassium-tantalum-heptafluoride. The lastmentioned salts must, of course, be reprocessed in order to recover thetantalum that was not converted to metal during electrolysis.

A newer procedure for the electrolytic production of reactive refractorymetals is disclosed and claimed in a copending application, Serial No.467,628, filed November 8, 1954, by D. H. Barbour and R. M. Sarla (nowabandoned). This procedure employs an electrolyte comprising highlypurified sodium chloride, and the appropriate complex fluoride ofpotassium and the desired metal. The advantage of utilizing sodiumchloride in such an electrolyte lies in the fact that chlorine insteadof fluorine is evolved at the cathode. In this manner, corrosion defectsare materially reduced. In the practice of the above-outlined method, ithas been necessary to operate on a batch basis, since accumulation ofthe potassium and sodium fluorides in the cell and exhaustion of thesodium chloride contained alter the character of the electrolysis.

It is accordingly the primary object of the instant invention to providea procedure which will permit practically continuous production of highmelting point reactive metals by fusion electrolysis.

Another object of the invention is to provide an electrolytic processfor the production of reactive metals in visibly crystalline form.

A further object of the invention is to provide a procedure whicheliminates the necessity of reprocessing and repurifying electrolyticconstituents.

A still further object of the invention is to provide a fusionelectrolysis procedure, which procedure operates more conveniently, moreefiectively, and at less cost than heretofore employed methods.

It is a still further object of the invention to provide a procedure offusion electrolysis which substantially eliminates polarization.

A more specific object of the invention is to provide an electrolyticprocess for the production of tantalum metal, which process provides aproduct having a purity of 99.95 percent or better without requiringfurther purification steps.

Viewed in its broadest aspects, the instant invention contemplates asemi-continuous electrolytic process for the production of tantalum,titanium, columbium, chromium and similar metals, which process employsa fused electrolyte containing principally sodium chloride and fluoride,to which may 'be added the halide of the desired reactive metal. Suchadditions of reactive metal halide are made periodically during theelectrolysis period, thereby prolonging the operation and providingbetter control thereof. The proportions of the chloride to fluoride areso adjusted in relation to the quantity of refractory metal chloride asto result in the apparent formation in situ of the complex alkali metal,refractory metal, double fluoride. The electrolysis need be interruptedonly when the cathode has been coated with such a thickness of depositedmetal that its physical size necessitates its removal.

In this manner a chemically continuous process is obtained, since allthe elements contained in the source material are removed from the cellduring operation. By employing a cell permitting a very rapidchange-over of cathode, the instant process may be made mechanicallycontinuous as Well.

The minimum relationship between alkali metal fluoride and the reactivemetal chloride is illustrated by the following equations:

The above equations illustrate only four of the possible metals ingroups IV, V and VI which may be electrolyzed according to the method ofthe present invention. Although the equations show the alkali metal assodium, other alkali metals may be substituted therefor.

A minimum initial composition of the electrolyte is shown on the righthand side of each of the above equations. Additional sodium chlorideother than that shown in the equations may be included in theelectrolyte. In general, it is preferable to have some excess over theindicated quantity in order to insure that no fluorine will be evolvedat any time during the electrolysis, paiticularly at the start. Suchaddition may also be desirable to adjust the melting point of theelectrolyte.

As the electrolytic action proceeds, additional quantities of therefractory metal chlorides are added to the fused baths in quantitiesequal to or less than the amount corresponding to the stoichiometricequivalent of metal removed by electrolysis. As indicated above, thisaction may be repeated as often as desired until the metal deposited onthe cathode has built up to the extent that it occupies such anundesirably large portion of the cell volume as to necessitate itsremoval. When such circum- For the sake of conciseness, the method ofthe invention will be described in detail with particular reference totantalum, it being understood that it is not so limited.

As an example of the practice of the invention, an electrolytic bathcomprising equal quantities of sodium chloride and sodium fluoride waspurified to remove all traces of moisture and air by a special hightemperature vacuum treatment method such as that disclosed and claimedin my copending application Serial No. 782,617, filed Dec. 24, 1958. Themixture was then fused in an electrolytic cell. 3.65 pounds of tantalumpentachloride were added to this electrolyte, and the mixtureelectrolyzed at the voltage of 4 volts and a current of 90 amperes for aperiod of 4.65 hours. At the end of this period 450 grams of tantalummetal were obtained. of tantalum pentachloride were next added to thecell, and electrolysis continued at a cell voltage of four volts, and anincreased current of 95 amperes. This operation produced 828 grams oftantalum metal after 8.1 hours. In addition to the tantalumpentachloride added during this second period, two pounds of purifiedsodium chloride were also introduced into the cell to change the ratioof sodium chloride to sodium fluoride from 50 to 50, to 56 to 44. At theconclusion of the second electrolytic period, 6 /2 pounds of tantalumpentachloride were added to the cell. The cell voltage was raised to 5volts, the current to 188 amperes. Proceeding under these conditions for6.1 additional hours, 1223 grams of tantalum metal were produced. Afourth addition of tantalum pentachloride (7.7 pounds) was made to thecell, and electrolysis continued at 6 volts and 232 amperes, to give1434 grams of tantalum metal. For the fifth electrolytic period, 6.61pounds of tantalum pentachloride were added, and the process continuedat an increased voltage of 7 volts, and an increased amperage of 365amperes. Under these conditions 1401 grams of tantalum metal wererecovered. In summary, a total of 27.55 pounds of tantalum pentachloridewas added for the five electrolytic periods, and a total of 11.75 poundsof tantalum was recovered. The metal produced had a hardness varyingbetween 84 and 147 on the Brinell scale. The current efiiciency was 74percent.

As another example of the practice of this invention, crystals ofcolumbium (niobium) were produced by the electrolysis of columbiumpentachloride in a fused electrolyte consisting of 72.5 percent ofsodium chloride and 27.5 percent of sodium fluoride vacuum meltedaccording to procedures disclosed and claimed in my copendingapplication Serial No. 427,886, filed May 5, 1954.

In the manner described in the preceding example, a total of 5.65 poundsof CbCl containing 882 grams of columbium metal, was added toapproximately 17 pounds of the base electrolyte at a cell temperature of800 C. The mass was electrolyzed at 5.5 volts and 330 amperes. A totalof 460 grams of metal containing coarse, needlelike crystals averaging 2to 4 millimeters in length and one millimeter in diameter were produced.

The procedure described above was repeated with TiCl to produce titaniummetal. 2.5 pounds of TiCL; was added to about 17 pounds of a mixtureconsisting of potassium chloride and sodium fluoride in an 80-20 ratio.An electrolyzing current of about 180 amperes at a voltage of '5 voltsmaintained the cell in a fused state at a temperature of about 750 C.The reaction proceeded for 8 hours to give 63 grams of titanium metal.

To the salt residue from the above run was added 0.44 pound of K TiFThis mixture was then electrolyzed at an amperage of 180 to 285 amperesfor 13 /2 hours to 355 pounds give 153 grams of titanium metal. Thecathodically deposited titanium had a Brinell hardness of 239.

In the same manner 3.2 pounds of TiCL; were added to 17 pounds of a50-50 mixture of sodium chloride and sodium fluoride. This mixture wasmaintained in a fused state by passing an electrolyzing current of 225to 400 amperes under a voltage range of 5 to 6% volts. The reactionwhich required 12 /2 hours, gave grams of titanium. This metal had ahardness of 226 on the Brinell scale.

Advantages of this process, in addition to those mentioned heretofore,including its semi-continuous character, arethat the metal is easilyremoved from the cathode, and that it deposits in the form of dendritictrees, the individual crystals of which average from 1 to 2 millimetersin diameter, with a length of 4 to 5 millimeters. Previously, reactivemetals have been produced as powders in which formthey are contaminatedeasily, since the ratio of surface area to mass is considerably higherthan in the visibly crystalline form.

The deposited metal requires only simple leaching with hot water toremove mechanically entrained electrolyte, following which step themetal may be dried and are melted in an inert atmosphere to form aningot. Since an alkali metal chloride is included as an essentialconstituent of the electrolyte, the fluorine produced is not evolved asa gas, but immediately replaces chlorine to form a fluoride.

What is claimed is:

1. In the electrolytic production of reactive refractory metals ofgroups IV, V and VI of the periodic table, wherein a chloride of theselected reactive refractory metal is electrolyzed in a fused bath, theimprovement which comprises employing a fused bath containingprincipally at least one simple alkali metal chloride and at least onesimple alkali metal fluoride.

2. In the electrolytic production of reactive refractory metals ofgroups IV, V and VI of the periodic table,

wherein the most highly oxidized chloride of the selected reactiverefractory metal is electrolyzed in a fused bath, the improvement whichcomprises employing a fused bath containing principally at least onesimple alkali metal chloride and at least one simple alkali metalfluoride.

3. A process for the production of a reactive refractory metal selectedfrom the group consisting of tantalum, columbium, titanium and zirconiumcomprising electrolyzing, in a fused bath containing principally atleast one simple alkali metal chloride and at least one simple alkalimetal fluoride, a refractory metal chloride respectively selected fromthe group consisting of tantalum pentachloride, columbium pentachloride,titanium tetrachloride and zirconium tetrachloride.

4. A process in accordance with claim 3, wherein said selected reactiverefractory metal is tantalum and said selected refractory metal chlorideis tantalum pentachloride.

5. A process in accordance with claim 3, wherein said selected reactiverefractory metal is columbium and said selected refractory metalchloride is columbium pentachloride.

6. A process in accordance with claim 3, wherein said selected reactiverefractory metal is titanium and said selected refractory metal chlorideis titanium tetrachloride.

7. A process in accordance with claim 3, wherein said selected reactiverefractory metal is zirconium and said selected refractory metalchloride is zirconium tetrachloride.

8. A semi-continuous process for the production of a reactive refractorymetal selected from groups IV, V and VI of the periodic tablecomprising, providing in an electrolytic cell having an anode and acathode a fused bath containing at least one simple alkali metalchloride and at least one simple alkali metal fluoride, passing anelectric current between said anode and cathode, making repeatedadditions tosaid fused bath of said selected reactive refractory metalchloride in an amount up to the stoichiometrical equivalent of metalremoved by electrolysis, until the volume of the pure, crystalline metaldeposited on the cathode necessitates its removal, and then inserting anew cathode and continuing said electrol- 37815.

9. A semi-continuous process for the production of a reactive refractorymetal selected from groups IV, V and VI of the periodic tablecomprising, providing in an electrolytic cell having an anode and acathode a fused bath containing at least one simple alkali metalchloride and at least one simple alkali metal fluoride, passing anelectric current between said anode and cathode, making repeatedadditions to said fused bath of the most highly oxidized metal chlorideof said selected reactive refractory metal in an amount up to thestoichiometrical equivalent of metal removed by electrolysis, until thevolume of the pure, crystalline metal deposited on the cathodenecessitates its removal, and then inserting a new cathode andcontinuing said electrolysis.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE ELECTROLYTIC PRODUCTION OF REACTIVE REFRACTORY METALS OFGROUPS IV, V, AND VI OF THE PERIODIC TABLE, WHEREIN A CHLORIDE OF THESELECTED REACTIVE REFRACTORY METAL IS ELECTROLYZED IN A FUSED BATH, THEIMPROVEMENT WHICH COMPRISES EMPLOYING A FUSED BATH CONTAININGPRINCIPALLY AT LEAST ONE SIMPLE ALKALI METAL CHLORIDE AND AT LEAST ONESIMPLE ALKALI METAL FLUORIDE.